Over 15% of the total energy consumption is used for refrigeration. Now, the commonly used gas compression refrigeration technology has Carnot cycle efficiency up to only about 25%, and the gas refrigerant used in gas compression refrigeration damages atmospheric ozone layer and induces greenhouse effect. Therefore, exploration of pollution-free and environment friendly refrigeration materials and development of novel refrigeration technologies with low energy consumption and high efficiency become very urgent in the whole world.
Magnetic refrigeration technology, as characterized by environment friendly, energy efficient, stable and reliable, has drawn great attention worldwide in recent years. Several types of giant magnetocaloric materials at room temperature and even high temperature zone were found successionally in US, China, Holland and Japan, which significantly increased the expectation for environment friendly magnetic refrigeration technology, e.g. Gd—Si—Ge, LaCaMnO3, Ni—Mn—Ga, La(Fe,Si)13-based compound, Mn—Fe—P—As, MnAs-based compound, etc. Common features of these novel giant magnetocaloric materials lie in that their magnetic entropy changes are all higher than that of the traditional magnetic refrigeration material Gd working around room temperature (R. T.), their phase-transition properties are of the first-order, most of them show strong magnetocrystalline coupling characteristics, and magnetic phase transition is accompanied with distinct crystalline structural transition. These novel materials also show different features. For example, Gd—Si—Ge is not only expensive but also requires further purification of the raw material while being prepared. And the raw materials used to prepare Mn—Fe—P—As and MnAs-based compound, etc. are toxic; NiMn-based Heusler alloy shows large hysteresis loss, and so on.
Among the several novel materials found in the past over ten years, La(Fe,Si)13-based compound is commonly accepted worldwide and has the highest potential for magnetic refrigeration application in a high temperature zone or even at R.T. This alloy has many characters shown as follows: the cost of its raw material is low; phase-transition temperature, phase-transition property and hysteresis loss may vary upon component adjustment; its magnetic entropy change around R.T. is higher than that of Gd by one fold. In the laboratories of many countries, La(Fe,Si)13-based magnetic refrigeration material has been used for prototype test, which proved its refrigerating capacity is better than that of Gd.
The investigation also showed that the phase-transition property of La(Fe,Si)13-based compounds varies with the adjustment of its components. For example, for the compounds with low Si amount, its phase-transition property is normally of the first-order in nature. Upon the increasing of Co content and rising of Curie temperature, the first-order nature of phase-transition property is weakened and gradually transmitted to the second order; hysteresis loss decreased gradually (no hysteresis loss for the second-order phase transition). However, due to the change of components and exchange interaction, the range of magnetocaloric effect was reduced in turn. Addition of Mn lowered the Curie temperature by impacting the exchange interaction; the first-order phase-transition property weakened; hysteresis loss decreased gradually; and the range of magnetocaloric effect was reduced in turn. In contrast, it was found that replacement of La with small rare earth magnetic atoms (e.g. Ce, Pr, Nd) can enhance the first-order phase-transition property; and increase hysteresis loss and the range of magnetocaloric effect. It was also found that introduction of interstitial atom (e.g. C, H, B, etc.) with small atomic radii can increase Curie temperature; and enable magnetocaloric effect to occur in a higher temperature zone. For instance, where the content of the interstitial atom H in molecular formula LaFe11.5Si1.5Hα increased from α=0 to α=1.8, the phase-transition temperature (peak temperature of magnetocaloric) was raised from 200K to 350K. It was expected that the first-order phase-transition La(Fe,Si)13-based compound showing a giant magnetocaloric effect can be used in actual magnetic refrigeration application, so as to achieve ideal refrigerating effect.
However, La(Fe,Si)13-based compounds (particularly, first-order phase-transition material) shows low compressive strength, fragile and poor corrosion resisting ability due to its strong magnetocrystalline coupling property (the intrinsic property of the material). Samples made from certain components have been cracked into pieces right after being made, and even pulverized naturally if being kept in air. Due to its fragility, the material, while used as a magnetic refrigeration material in a refrigeration cycle, is cracked into powder, which blocks the circulating path and thus reduces magnetic refrigeration efficiency and shorten refrigerator's lifetime.
Chinese patent application CN101755312A discloses a reactive sintered magnetic heat-exchanging material and a method for preparing the same. Said material comprises a (La1-aMa)(Fe1-b-cTbYc)13-d-based alloy prepared by the steps of mixing precursors or powders such as a La precursor, a Fe precursor and a Y precursor, etc.; compressing the mixture into a green body; sintering the green body at a temperature of 1000˜1200° C. for a period of 2˜24 hours to form a phase having a composition of (La1-aMa)(Fe1-b-eTbYc)13-d. Using such a ceramimetallurgical method, a La(Fe,Si)13-based magnetocaloric material can be manufactured into a working material shape satisfying the requirement of a magnetic refrigerator. For example, a La(Fe,Si)13-based room-temperature magnetocaloric material doped with Co, as normally having second-order phase-transition property (weak magnetocrystalline coupling, and magnetic phase transition accompanied with slower and weaker lattice expansion), can be manufactured by the ceramimetallurgical method into a working material shape satisfying the requirement of a sample machine. The resultant material processes certain compressive strength and shows no (or less) microcracks during the cyclic process. However, regarding a first-order phase-transition La(Fe,Si)13-based material (strong magnetocrystalline coupling, and magnetic phase transition accompanied with significant lattice expansion), the working material with a regular shape manufactured by the ceramimetallurgical method unavoidably shows microcracks or breaks during the cyclic process, which means an undesired mechanical property thereby restricts the application of the material.